Conversion of motion-image sequences between different temporal sampling rates such as film frame-rates or video field-rates is well known and is frequently necessary. It is commonly used in television and video systems to convert material between different acquisition, storage and distribution formats. The conversion is usually done by FIR filters that create values for new pixels from a weighed sum of existing pixel values. Motion compensated processing is now often used, in which motion vectors are derived and used to calculate which of the existing input pixels are most likely to represent a particular interpolated output pixel.
The growth in digital cinematography and synergistic developments in the art of high-definition television have resulted in the increasing use of ‘film-like’ frame rates, such as 24 and 25 frames per second. Most development work on conversion has been based on the conversion of ‘television-like’ temporal sampling rates, such as 50 or 60 fields per second. Algorithms developed for these higher temporal sampling rates, especially motion-compensated conversion algorithms, have been found to be challenged by the higher inter-frame differences that occur at 24 frames per second. The current invention provides a novel method of overcoming this challenge.
European patent EP 0 775 421 describes a method of conversion between closely-spaced frame rates (i.e. where the difference between the temporal sampling rates is less than about 2 Hz). A temporal interpolation mode is combined with a synchronisation mode, in which input images are re-timed without any temporal interpolation of pixel values. The choice of mode is controlled by the relative temporal phase between the input and output images. The advantage of this technique is that most images are re-timed and any artefacts due to temporal interpolation are limited to short time periods (typically of the order of 500 ms) which occur regularly as the input to output temporal phase cycles. This technique does nothing to hide any artefacts that are produced in the temporal interpolation mode—the mode is selected only on the basis of temporal phase.